The present invention generally relates to a process for producing a water-absorbent polysaccharide, a water-absorbent polysaccharide obtainable by this process, a water-absorbent polysaccharide, a composite, a process for production of a composite, a composite produced by this process, the use of the water-absorbent polysaccharides or of the composites and the use of polyphosphates.
Most of the absorption materials used today, which are able to absorb in a short time large quantities of liquids (water, urine), are primarily based upon slightly crosslinked synthetic polymers. These include, for example, polymers and co-polymers based upon acrylic acid or acrylamide, which are not based upon renewable materials and are insufficiently or not at all biologically degradable.
In the prior art, however, are described numerous water-absorbing polymers which are based upon polysaccharides and which are at least partially biodegradable. The raw materials for the production of superabsorbers based upon polysaccharides are, however, frequently water-soluble and must be converted into the water-insoluble form, in order to be able to use them as superabsorbers for hygiene applications.
EP 0 538 904 A1 and U.S. Pat. No. 5,247,072 describe superabsorbers based upon carboxyalkylpolysaccharides. In the process, the carboxyalkylpolysaccharide is dissolved in water and isolated by drying or precipitation and then thermally crosslinked via internal ester bridges by the reaction of the hydroxyl groups of the polysaccharide skeleton with the acidic carboxyl groups. Since this crosslinking reaction is very sensitive to small changes of the pH value, the temperature or the reaction duration, absorbers with widely varying absorption properties are obtained. The materials are characterized by a high absorption capacity under pressure, which, however, falls to a fraction of the original absorption properties within a few weeks, upon storage of the absorber.
In U.S. Pat. No. 5,550,189 are described absorbers based upon carboxyalkylpolysaccharides, in which the aging stability is improved by addition of multifunctional crosslinkers, such as, e.g. aluminium salts or citric acid. The production of the absorbers occurs from a common, homogeneous aqueous solution of carboxyalkylpolysaccharide and crosslinker, in which the components are present in low concentration, isolated together and then thermally crosslinked. The synthesis of these absorbers requires a high energy and time consumption, since the aqueous solutions are only of very low concentration. The improvement of the aging stability in the many exemplary embodiments does not correspond to the demands relevant in practice.
EP 855 405 A1 deals with the problem of the lacking aging stability of the absorption capacity of swellable starch maleates and proposes as solution an attachment of mercapto compounds to the double bond of the maleic acid substituents. The absorption behavior of the products, in particular under pressure, is very low.
In U.S. Pat. No. 4,952,550 the production of an absorber based upon carboxymethylcellulose is described, wherein the carboxymethylcellulose in water or organic solvent is treated with multivalent metal salts and a hydrophobizing component. A thermal crosslinking is not carried out. According to the disclosure, the gel blocking in these absorbers is reduced by the hydrophobizing component.
In the processes known from the prior art for crosslinking of polysaccharides, however, besides the partially low aging stability, it is observed that the homogeneous crosslinking of the polysaccharides hinders the biodegradability of the absorber, since the accessibility for microorganisms is reduced by the restricted swelling. Furthermore, in the crosslinking reactions known from the prior art, the enzymatic breakdown is inhibited by the additionally introduced substituents [Mehltretter et al., Journal of the American Oil Chemists Society, 47 (1970), pages 522-524].
In order to improve these disadvantageous properties, it was proposed to limit the crosslinking of the polysaccharide to the surface area, which, however, leads to products which do indeed have a satisfactory absorption under pressure, however, are frequently characterized by only an unsatisfactory absorption capacity under normal pressure and above all, caused by the restriction of the crosslinking to the surface area, by a low gel strength compared to homogeneously crosslinked polymers. Low gel strength leads to the formation of fine dust parts during processing processes, such as, for example, sieving or conveying, and thereby to health impacts of the workers involved in the production of the superabsorbers.
WO 02/096953 A1 describes a process for producing superabsorbers based upon surface-modified polycarboxypolysaccharides, in which an uncrosslinked polysaccharide is swollen with water to form a hydrogel, the hydrogel is then mechanically comminuted and dried and then the thus-obtained polymer particles are coated with a solution of a crosslinker and subjected to a surface crosslinking. Disadvantageous in the process described in WO 02/096953 A1 is, however, that in the formation of the hydrogel an organic solvent must be added to the water, in order to induce the swelling of the polysaccharide. The addition of the organic solvent however leads to the swollen polysaccharide being extremely “slimy”, which makes their further processing significantly more difficult. Furthermore, the organic solvents remain at least partially in the end product, which is questionable for ecological reasons. WO 00/21581 A1 also discloses a process in which gels made from crosslinked polysaccharides are brought into contact with organic solvents, in order to obtain absorbent polysaccharides with improved absorption properties. Disadvantageous in this process is the use of organic solvents.
U.S. Pat. No. 5,470,964 describes the production of an absorber based upon acid group-containing polysaccharides at the surface with multivalent metal ions, which has an improved absorption against pressure. Disadvantageous in this process is that for the improved absorption capacity of the absorber against pressure a relatively thick layer of the surface must be crosslinked and that according to the disclosure this is only possible with prior swelling of the polysaccharide with large quantities of solvent. In the swollen state the multivalent metal ions can then penetrate deeply enough into the surface. In order to achieve this, the polysaccharide is added to an excess of the aqueous metal salt solution, wherein the water excess lies in a 2-fold to 40-fold amount based upon the polysaccharide. By means of the thick crosslinked surface layer, good absorption values against pressure are indeed achieved, the free swell capacity as well as the retention capacity of the absorber is, however, disadvantageously reduced. It is further disadvantageous in the described process that the part of the polysaccharide added last to the crosslinker solution in the production process has available less swelling time and a reduced crosslinker concentration, so that an inhomogeneous distribution of the crosslinker results upon the surface, whereby wide variations of the absorption properties arise.
In general, one aspect of the present invention is to overcome the disadvantages arising from the state of the art. In particular, one aspect of the present invention provides biodegradable, superabsorbent polymers based upon renewable raw materials, which do not have the above described deficiencies. In particular, the absorber should have a high long-term storage stability, in which the absorption properties remain as far as possible. At the same time it is intended that the absorber particles have a high mechanical stability, in order to avoid the formation of fine dust parts during processing processes such as, for example, sieving or conveying.
Furthermore, regarding the absorption behavior, the absorbers should not tend to gel blocking, in particular in absorption layers comprising a lot of superabsorber (mostly more than about 65 wt % based upon the absorbent layer) and besides a high absorption and retention capacity also possess a high absorption capacity against pressure for water and aqueous solutions.
In absorbent layers or cores comprising a lot of superabsorber, and diapers comprising these, a wetting through characterized as leakage is often observed. This, and the gel blocking, are usually due to a slimy swollen hydrogel or at least to slimy components of the hydrogel. An object of this invention is thus to make available a less slimy hydrogel-forming absorbent polymer, which is suitable for use in hygiene articles. For a good absorption and application behavior it is necessary that the absorber has a predominantly insoluble character also in an excess of aqueous solution. Furthermore, the absorbers should be characterized by a particularly good biodegradability and be as free as possible from organic solvents.
A further aspect of the invention finds a production process for such superabsorbent polymers, which is simple, economical and can be reliably carried out, delivers a uniform product quality and in which small quantities of solvents are used and organic solvents are avoided if possible. Furthermore, it should be possible to carry out the process without the use of toxicologically questionable substances.
In addition, an aspect of the present invention consists in improving the biodegradability of hygiene articles such as sanitary napkins, wound dressings, incontinence articles and diapers.